Rubber composition for tire tread and pneumatic tire using the same

ABSTRACT

A rubber composition for a tire tread is disclosed, which provides a tire excellent in wet grip performance at low temperatures and wet grip performance at high temperatures. The rubber composition for a tire tread includes, per 100 parts by mass of diene-based rubbers including (a) 25 to 65 parts by mass of a modified styrene butadiene rubber having a glass transition point of −30° C. or more and (b) 25 to 65 parts by mass of a modified styrene butadiene rubber having a glass transition point of −50° C. or less: 80 to 140 parts by mass of silica; 15 to 50 parts by mass of a thermoplastic resin having a softening point of 40° C. or more; and 5 to 15 parts by mass of an oil.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a rubber composition for a tire tread and also to a pneumatic tire using the same.

2. Description of Related Art

In rubber compositions used for tires, there is a demand to improve the grip performance on wet road surfaces (wet grip performance).

In order to address such a problem, JP2007-321046A describes that a styrene butadiene rubber having a glass transition point of −30 to 0° C. is used as a rubber component. In addition, JP3636546B and JP5902583B describe that a high-softening-point resin is used to improve grip performance. However, there still has been room for improvement in wet grip performance at low temperatures.

SUMMARY OF THE INVENTION

In light of the above points, an object of an aspect of the invention is to provide a rubber composition that provides a pneumatic tire excellent in wet grip performance at low temperatures (5° C.) and wet grip performance at high temperatures (25° C.)

The invention encompasses the following embodiments.

[1] A rubber composition for a tire tread, including, per 100 parts by mass of diene-based rubbers including (a) 25 to 65 parts by mass of a modified styrene butadiene rubber having a glass transition point of −30° C. or more and (b) 25 to 65 parts by mass of a modified styrene butadiene rubber having a glass transition point of −50° C. or less: 80 to 140 parts by mass of silica; 15 to 50 parts by mass of a thermoplastic resin having a softening point of 40° C. or more; and 5 to 15 parts by mass of an oil.

[2] The rubber composition for a tire tread according to [1], wherein the difference in styrene content between the modified styrene butadiene rubber (a) and the modified styrene butadiene rubber (b) is 5 to 30 mass %.

[3] The rubber composition for a tire tread according to [1] or [2], wherein the thermoplastic resin is a styrene-based resin or a terpene-based resin.

[4] The rubber composition for a tire tread according to any one of [1] to [3], having a storage modulus of 150 MPa or less as measured at −5° C., a frequency of 10 Hz, a dynamic strain of 0.2%, and a static strain of 10%.

[5] The rubber composition for a tire tread according to any one of [1] to [4], wherein the difference between the storage modulus (A) measured at 25° C., a frequency of 10 Hz, a dynamic strain of 0.1%, and a static strain of 10% and the storage modulus (B) measured at 25° C., a frequency of 10 Hz, a dynamic strain of 5%, and a static strain of 10% is 20 MPa or less.

[6] The rubber composition for a tire tread according to any one of [1] to [5], including 5 to 20 parts by mass of a protected mercaptosilane per 100 parts by mass of silica.

[7] A pneumatic tire using the rubber composition according to any one of [1] to [6] in a tread thereof.

According to a rubber composition for a tire tread of an aspect of the invention, a pneumatic tire excellent in wet grip performance at low temperatures (5° C.) and wet grip performance at high temperatures (25° C.) can be obtained.

DESCRIPTION OF EMBODIMENTS

Hereinafter, matters relevant to the practice of the invention will be described in detail.

A rubber composition for a tire tread according to this embodiment includes, per 100 parts by mass of diene-based rubbers including (a) 25 to 65 parts by mass of a modified styrene butadiene rubber (SBR) having a glass transition point (Tg) of −30° C. or more and (b) 25 to 65 parts by mass of a modified styrene butadiene rubber (SBR) having a glass transition point (Tg) of −50° C. or less: 80 to 140 parts by mass of silica; 15 to 50 parts by mass of a thermoplastic resin having a softening point of 40° C. or more; and 5 to 15 parts by mass of an oil. As used herein, the glass transition point (Tg) is a value measured in accordance with JIS K7121 by a differential scanning calorimetry (DSC) method at a temperature rise rate of 20° C./min (measurement temperature range: −150° C. to 150° C.). The softening point is a value measured by the ring and ball method in accordance with JIS K2207.

As used herein, the term “modified” in a rubber component means having a functional group reactive with silica, and the term “unmodified” means having no functional group reactive with silica. As functional groups reactive with silica, for example, a hydroxyl group, an amino group, a carboxyl group, an alkoxy group, an alkoxysilyl group, an epoxy group, and the like can be mentioned. Such a functional group may be introduced at the molecular terminal or may alternatively be introduced into the molecular chain.

The diene-based rubbers may also include components other than the modified SBR (a) and the modified SBR (b). As such a rubber component, a diene-based rubber having a Tg of −30° C. or less is preferable. For example, natural rubbers (NR), synthetic isoprene rubbers (IR), butadiene rubbers (BR), styrene butadiene rubbers (SBR), nitrile rubbers (NBR), chloroprene rubbers (CR), butyl rubbers (IIR), styrene-isoprene copolymer rubbers, butadiene-isoprene copolymer rubbers, styrene-isoprene-butadiene copolymer rubbers, and the like are usable. They may be modified or unmodified, but unmodified SBRs are preferable. That is, unmodified SBRs having a Tg of −30° C. or less are more preferable.

The proportion of the total content of the modified SBR (a) and the modified SBR (b) in the diene-based rubbers is preferably 50 to 100 mass %, preferably 60 to 90 mass %, and more preferably 60 to 80 mass %.

The content ratio between the modified SBR (a) and the modified SBR (b) (modified SBR (a)/modified SBR (b)) is not particularly limited, but is, on a mass basis, preferably 0.4 to 2.6, and more preferably 0.6 to 1.7.

The difference in styrene content between the modified SBR (a) and the modified SBR (b) is preferably 5 to 30 mass %, and more preferably 5 to 20 mass %. When the difference in styrene content is within the above range, excellent wet grip performance is likely to be obtained.

The rubber composition according to this embodiment contains silica as a reinforcing filler. Silica is not particularly limited, and, for example, wet silica such as wet-precipitated silica or wet-gelled silica may be used.

The silica content is, per 100 parts by mass of the diene-based rubbers, 80 to 140 parts by mass, preferably 100 to 140 parts by mass, and more preferably 110 to 130 parts by mass.

As the reinforcing filler, in addition to silica, carbon black may also be used in combination. The reinforcing filler content (the total amount of silica and carbon black) is not particularly limited and may be, per 100 parts by mass of the diene-based rubbers, 80 to 170 parts by mass, 100 to 160 parts by mass, or 110 to 150 parts by mass. The carbon black content is not particularly limited either and may be, per 100 parts by mass of the diene-based rubbers, 1 to 30 parts by mass, or 5 to 20 parts by mass.

The rubber composition according to this embodiment preferably contains a silane coupling agent. In that case, the silane coupling agent content is, per 100 parts by mass of silica, preferably 5 to 20 parts by mass, and more preferably 5 to 15 parts by mass. In addition, the silane coupling agent content is, per 100 parts by mass of the diene-based rubbers, preferably 4 to 30 parts by mass, and more preferably 5 to 20 parts by mass.

As silane coupling agents, sulfide silanes such as bis(3-triethoxysilylpropyl)tetrasulfide (e.g., “Si69” manufactured by Degussa AG), bis(3-triethoxysilylpropyl) disulfide (e.g., “Si75” manufactured by Degussa AG), bis(2-triethoxysilylethyl)tetrasulfide, bis(4-triethoxysilylbutyl)disulfide, bis (3-trimethoxysilylpropyl)tetrasulfide, and bis (2-trimethoxysilylethyl)disulfide, mercaptosilanes such as γ-mercaptopropyltrimethoxysilane, γ-mercaptopropyltriethoxysilane, mercaptopropylmethyldimethoxysilane, mercaptopropyldimethylmethoxysilane, and mercaptoethyltriethoxysilane, and protected mercaptosilanes such as 3-octanoylthio-1-propyltriethoxysilane and 3-propionylthiopropyltrimethoxysilane can be mentioned. Among them, protected mercaptosilanes are preferable.

The rubber composition according to this embodiment contains a thermoplastic resin having a softening point of 40° C. or more, and the softening point is preferably 40 to 160° C. As a result of containing a thermoplastic resin having a softening point of 40° C. or more, excellent wet grip performance is likely to be obtained.

The thermoplastic content is, per 100 parts by mass of the diene-based rubbers, 15 to 50 parts by mass, preferably 20 to 40 parts by mass. When the thermoplastic resin content is within the above range, excellent wet grip performance is likely to be obtained.

As thermoplastic resins, styrene-based resins, terpene-based resins, petroleum-based hydrocarbon resins, rosin-based resins, and the like can be mentioned, and, among them, styrene-based resins and terpene-based resins are preferable.

Styrene-based resins may be resins containing styrene and/or α-methylstyrene as a constituent monomer, examples thereof including homopolymers obtained by polymerizing styrene or α-methylstyrene alone, copolymers obtained by copolymerizing styrene and α-methylstyrene, and copolymers of styrene and/or α-methylstyrene with other monomers. As other monomers, for example, terpene compounds such as α-pinene, β-pinene, dipentene, limonene, myrcene, alloocimene, ocimene, α-phellandrene, α-terpinene, γ-terpinene, terpinolene, 1,8-cineole, 1,4-cineol, α-terpineol, β-terpineol, and γ-terpineol (terpene-based monomers), non-conjugated olefins such as ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, and 1-octene, and the like can be mentioned. They may be used alone, and it is also possible to use two or more kinds in combination.

As terpene-based resins, for example, terpene-based resins such as an α-pinene polymer, a β-pinene polymer, and a dipentene polymer, modified terpene-based resins obtained by modifying these terpene-based resins (phenol modification, aromatic modification, hydrocarbon modification, etc.) (e.g., terpene phenol-based resins, styrene-modified terpene-based resins, aromatic-modified terpene-based resins, etc.), and the like can be mentioned.

As petroleum-based hydrocarbon resins, for example, C5-based aliphatic hydrocarbon resins, C9-based aromatic hydrocarbon resins, and C5/C9-based aliphatic/aromatic copolymer hydrocarbon resins can be mentioned. An aliphatic hydrocarbon resin is a resin obtained by the cationic polymerization of an unsaturated monomer such as isoprene or cyclopentadiene, which is a petroleum fraction equivalent to four to five carbon atoms (C5 fraction), and may also be partially hydrogenated. An aromatic hydrocarbon resin is a resin obtained by the cationic polymerization of a monomer such as vinyltoluene, an alkylstyrene, or indene, which is a petroleum fraction equivalent to eight to ten carbon atoms (C9 fraction), and may also be partially hydrogenated. An aliphatic/aromatic copolymer hydrocarbon resin is a resin obtained by copolymerizing the above C5 and C9 fractions by cationic polymerization, and may also be partially hydrogenated.

As rosin-based resins, for example, raw material rosins such as gum rosin, wood rosin, and tall oil rosin, disproportionated products of raw material rosins, polymerized rosins, and like rosins, esterified products of rosins (rosin ester resins), phenol-modified rosins, unsaturated acid-(e.g., maleic acid-) modified rosins, formylated rosins obtained by reduction-treating rosins, and the like can be mentioned.

As an oil, any of various oils generally blended into rubber compositions can be used. For example, a mineral oil, that is, at least one mineral oil selected from the group consisting of a paraffinic oil, a naphthenic oil, and an aromatic oil, may be used. The oil content is, per 100 parts by mass of the diene-based rubbers, 5 to 15 parts by mass, preferably 5 to 10 parts by mass.

In addition to the above components, the rubber composition according to this embodiment may have blended therein various additives generally used in rubber compositions, such as zinc oxide, stearic acid, antioxidants, waxes, vulcanizing agents, and vulcanization accelerators.

A preferred example of the vulcanizing agents is sulfur. The vulcanizing agent content is not particularly limited, but is, per 100 parts by mass of the rubber component, preferably 0.1 to 10 parts by mass, and more preferably 0.5 to 5 parts by mass. In addition, as the vulcanization accelerators, for example, sulfenamide-based, thiuram-based, thiazole-based, guanidine-based, and like various vulcanization accelerators can be mentioned. They can be used alone, and it is also possible to use a combination of two or more kinds. The vulcanization accelerator content is not particularly limited, but is, per 100 parts by mass of the rubber component, preferably 0.1 to 7 parts by mass, and more preferably 0.5 to 5 parts by mass.

For the reason that excellent wet grip performance can be obtained, it is preferable that the rubber composition according to this embodiment has a storage modulus of 150 MPa or less as measured at −5° C., a frequency of 10 Hz, a dynamic strain of 0.2%, and a static strain of 10%.

For the reason that excellent wet grip performance can be obtained, it is preferable that in the rubber composition according to this embodiment, the difference between the storage modulus (A) measured at 25° C., a frequency of 10 Hz, a dynamic strain of 0.1%, and a static strain of 10% and the storage modulus (B) measured at 25° C., a frequency of 10 Hz, a dynamic strain of 5%, and a static strain of 10% is 20 MPa or less.

The rubber composition according to this embodiment can be made by kneading in the usual manner using a commonly used mixer, such as a Banbury mixer, a kneader, or a roll. That is, for example, in the first mixing stage, additives excluding a vulcanizing agent and a vulcanization accelerator are added to diene-based rubbers and mixed, and then, in the final mixing stage, a vulcanizing agent and a vulcanization accelerator are added to the obtained mixture and mixed. As a result, a rubber composition can be prepared.

The rubber composition thus obtained is applicable to the treads of pneumatic tires of various sizes for various applications, including tires for passenger cars, large-sized tires for trucks and buses, and the like. That is, in the usual manner, the rubber composition is formed into a predetermined shape by extrusion, for example, and combined with other parts to make a green tire. Subsequently, the green tire is vulcanization-molded at 140 to 180° C., for example, whereby a pneumatic tire can be produced.

EXAMPLES

Hereinafter, examples of the invention will be shown, but the invention is not limited to these examples.

Using a lab mixer, following the formulations (parts by mass) shown in Table 1 below, first, in the first mixing stage, ingredients excluding sulfur and a vulcanization accelerator were added to diene-based rubber components and kneaded (discharge temperature: 160° C.). Next, in the final mixing stage, sulfur and a vulcanization accelerator were added to the obtained kneaded product and kneaded (discharge temperature: 90° C.), thereby preparing a rubber composition. The details of the components in Table 1 are as follows.

-   -   Unmodified SBR 1: “SL 563” manufactured by JSR Corporation,         Tg=−35° C.     -   Unmodified SBR 2: “SBR 1502” manufactured by JSR Corporation,         Tg=−52° C.     -   Unmodified SBR 3: “TUF 1834” manufactured by Asahi Kasei         Corporation, Tg=−70° C.     -   Modified SBR 1: “HPR 850” manufactured by JSR Corporation,         Tg=−24° C., styrene content=27 mass %     -   Modified SBR 2: “HPR 840” manufactured by JSR Corporation,         Tg=−60° C., styrene content=10 mass %     -   Carbon black: “SEAST KH” manufactured by Tokai Carbon Co., Ltd.     -   Silica: “Nipsil AQ” manufactured by Tosoh Silica Corporation     -   Silane coupling agent: “NXT” manufactured by Momentive         Performance Materials, protected mercaptosilane     -   Oil: “PROCESS NC140” manufactured by ENEOS Corporation     -   Thermoplastic resin 1: C5/C9-based resin, “Petrotack 90”         manufactured by Tosoh Corporation, softening point=100° C.     -   Thermoplastic resin 2: Styrene-based resin, “SYLVATRAXX 4401”         manufactured by Kraton, softening point=85° C.     -   Thermoplastic resin 3: Terpene-based resin, “SYLVATRAXX 4150”         manufactured by Kraton, softening point=115° C.     -   Zinc oxide: “Type 2 Zinc Oxide” manufactured by Mitsui Mining &         Smelting Co., Ltd.     -   Stearic acid: “LUNAC S-20” manufactured by Kao Corporation     -   Wax: “OZOACE 0355” manufactured by Nippon Seiro Co., Ltd.     -   Antioxidant 1: “NOCRAC 6C” manufactured by Ouchi Shinko Chemical         Industrial Co., Ltd.     -   Antioxidant 2: “ANTAGE RD” manufactured by Kawaguchi Chemical         Industry Co., Ltd.     -   Vulcanization accelerator 1: “NOCCELER D” manufactured by Ouchi         Shinko Chemical Industrial Co., Ltd.     -   Vulcanization accelerator 2: “SOXINOL CZ” manufactured by         Sumitomo Chemical Co., Ltd.     -   Sulfur: “Powder Sulfur” manufactured by Tsurumi Chemical         Industry Co., Ltd.

Each obtained rubber composition was vulcanized at 160° C. for 20 minutes to make a test piece having a predetermined shape, and a dynamic viscoelasticity test was performed. In addition, using each obtained rubber composition, a pneumatic tire was made and evaluated for wet grip performance. The measurement methods are as follows.

-   -   Storage Modulus at −5° C. (E′ (−5° C.)): Using a viscoelasticity         tester manufactured by GABO, the storage modulus was measured at         −5° C., a frequency of 10 Hz, a dynamic strain of 0.2%, and a         static strain of 10%.     -   Difference between Storage Modulus (A) Measured at 25° C., 10-Hz         Frequency, 0.1% Dynamic Strain, and 10% Static Strain and         Storage Modulus (B) measured at 25° C., 10-Hz Frequency, 5%         Dynamic Strain, and 10% Static Strain (ΔE′ (25° C.)): The         storage modulus was measured using a viscoelasticity tester         manufactured by GABO, and the difference between the storage         modulus (A) and the storage modulus (B) was determined.     -   Wet Grip Performance (5° C.): Four 215/45ZR17 test radial tires         having the rubber composition obtained above in the tread were         mounted on a vehicle. Under 5° C. temperature conditions, the         vehicle was run on a road surface with a water depth of 1 mm at         a speed of 100 km/h, and then the ABS was activated. The braking         distance at this time was measured, and the reciprocal of the         measured value was calculated. The result was evaluated as an         index, taking the result in Comparative Example 1 as 100. A         larger index indicates better wet grip performance.     -   Wet Grip Performance (25° C.): Four 215/45ZR17 test radial tires         having the rubber composition obtained above in the tread were         mounted on a vehicle. Under 25° C. temperature condition, the         vehicle was run on a road surface with a water depth of 1 mm at         a speed of 100 km/h, and then the ABS was activated. The braking         distance at this time was measured, and the reciprocal of the         measured value was calculated. The result was evaluated as an         index, taking the result in Comparative Example 1 as 100. A         larger index indicates better wet grip performance.

TABLE 1 Compar- Compar- Compar- Compar- Compar- ative ative ative ative ative Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- Exam- ple 1 ple 2 ple 3 ple 4 ple 5 ple 1 ple 2 ple 3 ple 4 ple 5 ple 6 ple 7 Unmodified 50 — 20 20 — — — — 20 — — — SBR 1 Unmodified 50 — — — — — — 20 — — — — SBR 2 Unmodified — 20 — — — 20 20 — 20 35 20 20 SBR 3 Modified — 20 — 60 80 50 50 30 30 30 50 50 SBR 1 Modified — 60 80 20 20 30 30 50 30 35 30 30 SBR 2 Carbon 8 8 8 8 8 8 8 8 8 8 8 8 black Silica 120 120 120 120 120 120 120 120 120 120 120 120 Silane 10 10 10 10 10 10 10 10 10 10 10 10 coupling agent Oil 35 10 10 10 10 10 10 10 10 10 10 10 Thermoplastic 15 — — — — — — — — — — 30 resin 1 Thermoplastic — — 30 — — — 30 30 30 — 17 — resin 2 Thermoplastic — 30 — 30 30 30 — — — 17 — — resin 3 Zinc 3 3 3 3 3 3 3 3 3 3 3 3 oxide Stearic 2 2 2 2 2 2 2 2 2 2 2 2 acid Wax 2 2 2 2 2 2 2 2 2 2 2 2 Antioxidant 1 2 2 2 2 2 2 2 2 2 2 2 2 Antioxidant 2 2 2 2 2 2 2 2 2 2 2 2 2 Vulcanization 2 2 2 2 2 2 2 2 2 2 2 2 accelerator 1 Vulcanization 2 2 2 2 2 2 2 2 2 2 2 2 accelerator 2 Sulfur 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 E′ (−5° C.) 120 70 105 165 160 95 90 35 140 80 105 115 ΔE′ (25° C.) 25 11.5 12 12.5 10 11.5 12 14 18 17 10.8 13 Wet grip 100 108 107 94 97 105 106 108 102 104 102 103 performance (5° C.) Wet grip 100 94 92 107 106 107 108 104 104 104 105 104 performance (25° C.)

The results are as shown in Table 1. Examples 1 to 7 were superior to Comparative Example 1 in wet grip performance at low temperatures and wet grip performance at high temperatures.

Comparative Examples 2 and 3 are examples where the modified SBR (a) content is less than the lower limit, and was inferior to Comparative Example 1 in wet grip performance at high temperatures.

Comparative Examples 4 and 5 are examples where the modified SBR (b) content is less than the lower limit, and the wet grip performance at low temperatures was inferior.

The rubber composition of the invention can be used as a rubber composition for various tires for passenger cars, light trucks, buses, and the like. 

What is claimed is:
 1. A rubber composition for a tire tread, comprising, per 100 parts by mass of diene-based rubbers including (a) 25 to 65 parts by mass of a modified styrene butadiene rubber having a glass transition point of −30° C. or more and (b) 25 to 65 parts by mass of a modified styrene butadiene rubber having a glass transition point of −50° C. or less: 80 to 140 parts by mass of silica; 15 to 50 parts by mass of a thermoplastic resin having a softening point of 40° C. or more; and 5 to 15 parts by mass of an oil.
 2. The rubber composition for a tire tread according to claim 1, wherein the difference in styrene content between the modified styrene butadiene rubber (a) and the modified styrene butadiene rubber (b) is 5 to 30 mass %.
 3. The rubber composition for a tire tread according to claim 1, wherein the thermoplastic resin is a styrene-based resin or a terpene-based resin.
 4. The rubber composition for a tire tread according to claim 2, wherein the thermoplastic resin is a styrene-based resin or a terpene-based resin.
 5. The rubber composition for a tire tread according to claim 1, having a storage modulus of 150 MPa or less as measured at −5° C., a frequency of 10 Hz, a dynamic strain of 0.2%, and a static strain of 10%.
 6. The rubber composition for a tire tread according to claim 2, having a storage modulus of 150 MPa or less as measured at −5° C., a frequency of 10 Hz, a dynamic strain of 0.2%, and a static strain of 10%.
 7. The rubber composition for a tire tread according to claim 1, wherein the difference between the following storage modulus (A) and storage modulus (B) is 20 MPa or less: Storage modulus (A): measured at 25° C., a frequency of 10 Hz, a dynamic strain of 0.1%, and a static strain of 10%, Storage modulus (B): measured at 25° C., a frequency of 10 Hz, a dynamic strain of 5%, and a static strain of 10%.
 8. The rubber composition for a tire tread according to claim 2, wherein the difference between the following storage modulus (A) and storage modulus (B) is 20 MPa or less: Storage modulus (A): measured at 25° C., a frequency of 10 Hz, a dynamic strain of 0.1%, and a static strain of 10%, Storage modulus (B): measured at 25° C., a frequency of 10 Hz, a dynamic strain of 5%, and a static strain of 10%.
 9. The rubber composition for a tire tread according to claim 1, comprising 5 to 20 parts by mass of a protected mercaptosilane per 100 parts by mass of silica.
 10. The rubber composition for a tire tread according to claim 2, comprising 5 to 20 parts by mass of a protected mercaptosilane per 100 parts by mass of silica.
 11. A pneumatic tire using the rubber composition according to claim 1 in a tread thereof.
 12. A pneumatic tire using the rubber composition according to claim 2 in a tread thereof. 